The present invention relates generally to the ramjet engine and, in particular, to an improved fixed geometry-type ramjet engine that is compatible to both supersonic and hypersonic flight.
The ramjet is the simplest of all air-breathing propulsion engines used to produce supersonic and/or hypersonic flight within the earth's atmosphere. Although it is self-evident that the fixed geometry ramjet engine offers the most efficient operation at a particular and relatively narrow flight regime, since, obviously, the specific engine design and, in particular, its combustor and exhaust nozzle can be tailor-made to a selected operating speed regime, it is a conventional practice to utilize a single combustion chamber with a variable, two-position exhaust nozzle whereby the size and to a certain extent the configuration of the nozzle and the area of the throat may be altered to one of two different settings to thus produce a wider operating speed range, or, in other words, make the same ramjet engine capable of operating at both supersonic and hypersonic speeds merely by changing the position of the nozzle to vary the area of the throat.
Initially, of course, the ramjet engine and vehicle in which housed would be required to be boosted or accelerated preferably to at least a low supersonic speed needed for its relatively efficient operation. This boost could be provided in the case of a ramjet-powered missile, for example, which is perhaps the most desirable use of the ramjet, either by the ground-or air-launch thereof. During such launching operation, the exhaust nozzle of the conventional, variable geometry ramjet would have been adjusted to its first position of adjustment compatible with a predesigned supersonic speed range. Upon completion of the aforesaid launching operation, combustion would then be initiated in the single combustion chamber with the said variable geometry exhaust nozzle having been already adjusted to the aforementioned first position of adjustment. This action would naturally accelerate the previously launched ramjet-powered missile, for example, to a relatively high supersonic speed. At this time, the exhaust nozzle would then be changed to its second position of adjustment to even further accelerate the missile into the hypersonic speed regime.
Although the two-position nozzle offers an opportunity to extend the inherent capability of the ramjet to operate in a much wider flight speed range, it also suffers from the obvious disadvantage of requiring a rather complex mechanical arrangement to adjust the position of the nozzle from one flight regime to the other and which may also add undesirable weight to the system. It is, therefore, clearly very desirable, if not absolutely essential, to produce a ramjet engine that both inherently combines the superior performance level of a wide speed range, supersonic/hypersonic ramjet that is available with the previously-referred to two-position exhaust nozzle, with the relative simplicity and efficiency of a fixed geometry ramjet engine. In this connection the new and improved ramjet of the present invention proposes the solution to the above-outlined problem by mounting two separate, fixed geometry stages; namely, a boost stage and a cruise stage in tandem with the boost stage being ejected after performing the requisite acceleration to the speed at which combustion of the cruise stage would be initiated. In this connection, it is to be noted that the broad concept of the use of an ejectable element is already known, for example, with specific regard to an "afterburner-booster", described in a Marquardt Aircraft Co. report, entitled "Study of Self-Acceleration Potentialities of Ramjet Engines", dated May 22, 1951, in which there was utilized an "open tailpipe as an ejectable afterburner"; however, the design of that arrangement was limited to acceleration from subsonic speeds to low supersonic speeds. On the other hand, as will be noted hereinafter in the following summary and detailed description, the present invention is specifically designed to significantly increase the efficiency of the ramjet engine over a much wider speed range from low supersonic to hypersonic with the use of two, separate and principal, thrust-producing stages arranged in tandem and which are obviously quite different from the above-referred to previously-proposed "ejectable afterburner". In this regard, a U.S. Pat. No. 3,040,517, issued on June 26, 1962 to C. V. Ryden et al involved a "Releasable Rocket Nozzle" in which a booster nozzle is made releasable from a ramjet vehicle. In particular, the Ryden arrangement consists of a ramjet vehicle having both a ramjet motor and a rocket booster motor mounted in tandem relative to each other and respectively exhausting through their own individual exhaust nozzle. When the boost operation of the Ryden ramjet vehicle is completed, the exhaust nozzle for the boost operation is then released and thus the obvious and well-known relative inefficiencies of a single nozzle for both the booster and ramjet used in previously-developed designs involving the combination of ramjet and rocket motors is thus eliminated. However, the two-stage ramjet arrangement of the present invention constitutes an advantageous technique by involving the use of completely separate combustor-nozzle assemblies that ensures the utilization of the most optimum combustion chamber design, as well as the exhaust nozzle design, for each of two speed ranges to thus produce a still further improvement in the field of ramjets, as will become readily apparent hereinafter in the following disclosure.